The present invention is in the field of dehydrogenation catalysts using a modified sol-gel/co-precipitation technique.
Over the past 15 years, many academic and industrial research efforts have focused on the conversion of lower alkanes (C1-C5) to petrochemical feed stocks. These catalytic reactions include: methane partial oxidation to formaldehyde and oxidative coupling to C2 hydrocarbons; ethane and propane oxidative dehydrogenation to olefins and oxygenates (acetic acid, acrolein, acrylic acid); and butane and pentane oxidative dehydrogenation to maleic and phthalic anhydrides. Thus far, there are no industrially practical operations for such applications except for the production of maleic anhydride from butane.
The research drive to develop an oxidative dehydrogenation process for propane and ethane comes from the fact that the chemical industry depends heavily on propene and other alkene feed stocks. For example, propene demand is estimated to grow 4.5% per year between 1991 and 2000. Catalytic oxidative dehydrogenation (ODH) is an attractive alternate route for the production of alkenes compared to conventional cracking and dehydrogenation processes. This is because ODH is thermodynamically favored at lower temperatures and usually does not lead to the formation of coke and smaller hydrocarbons. Recent literature has focused on selective, high surface area catalysts active below 823K that can limit the amount of carbon oxides formed. In our recent work, an observed yield of propene of 30% was obtained in the oxidative dehydrogenation of propane. This is among the highest yields ever reported for this type of reaction. Furthermore, certain formulations of the catalyst lead to little or no carbon monoxide production. These facts make this catalyst a viable option for an industrial process.
Some of the most selective oxidative dehydrogenation catalysts reported in, recent literature consist of vanadium and molybdenum compounds. In particular, promising results have been obtained when molybdate-based catalysts are promoted or supported. For example, Nixe2x80x94Coxe2x80x94Mo, Vxe2x80x94Nbxe2x80x94Mo/TiO2, Kxe2x80x94MnMoO4, and K2MoO4 have shown promise in ODH and other partial oxidation reactions. The positive effect of alkali dopants (Li, Na, K, Rb, and Cs) has been discussed in many oxidation reactions and is becoming more and more applicable to different catalysts. However, the effect is still not well characterized. Alkali doping can have the effect of increasing selectivity and activity while preventing phase transformations, inhibiting sintering, and creating basic centers on the catalyst surface. Abello et al. in Catal. Letters 53, 53 (1998) have shown a significant increase in selectivity on Mo/MgO-xcex3-Al2O3 with the addition of potassium. On this catalyst, an interesting trend was noticed in catalyst activity, redox behavior, and surface acidity. Furthermore, past work from our group has shown that potassium can largely affect oxygen exchange between bulk MnMoO4 catalysts and gas phase oxygen as well as adsorption/desorption behavior of the catalyst. These parameters are the most common features used to describe ODH catalysts.
As previously mentioned, the study of silica-titania mixed oxides has gained much attention because of their high activity for epoxidation reactions of olefins with hydroperoxides. It has been cited that TiO2 in mixed oxides of silica and titania can be present not only as anatase, but in the form of very small domains in which the normal octahedral coordination of TiO2 has changed to tetrahedral (see Notari, B., Adv. Catal. 41, 253 (1996)). This leads to the unique structural and chemical properties of this material. Silica-titania mixed oxide supports, through sol-gel preparations, can provide advantages that the respective single oxides cannot. These benefits include stronger metal-support interactions, hindering reduction of the active metal, and smaller particle size that leads to better dispersion and higher surface area. Silica-titania mixed oxides have been studied extensively for attributes such as acidity, porosity, Tixe2x80x94Oxe2x80x94Si connectivity, and phase separations. However, few studies have been done on their use as active metal supports.
Baiker et al. in Appl. Catal A 35, 365 (1987) and Vogt et al. in J. Catal. 114, 313 (1988) have used vanadia supported on silica-titania mixed oxides for the reduction of nitric oxide with ammonia. Baiker et al. has shown that the addition of titania causes an interaction that prevents agglomeration of surface vanadia species. Udomsak et al. in Ind. Eng. Chem. Res. 35, 47 (1996) have shown a significant difference in isobutane dehydrogenation activity on chromia/silica-titania catalyst with different preparation methods. Hydrogen and carbon monoxide interaction with titania promoted palladium on silica was studied by Rieck and Bell in J. Catal. 99, 262 (1986). Here, it was shown that TiO, species decorate the palladium, causing a notable difference in the CO adsorption behavior. Feng et al. in J. Catal, 136, 423 (1993) have shown the hydrogen abstracting ability of the weakly acidic silica titania mixed oxide supported palladium catalysts was the dominating factor for non-oxidative dehydrogenation of propane.
Sol-gel science is well summarized well by Brinker and Scherer in xe2x80x9cSol-Gel Sciencexe2x80x9d, Academic Press, New York, 1990, but the use of sol-gel preparations for supported metal catalysts is sparse. The use of the technology combined with active metal dispersion is limited to conventional techniques, such as wet impregnation (making the Si:Ti support first, then dispersing active components). Combining the positive effects of alkali doping and sol-gel science in a way to disperse the active component as the support network is forming has never been attempted in known literature.
Study of silica-titania mixed oxides have gained much attention because of their high activity for epoxidation reactions of olefins with hydroperoxides. Silica-titania mixed oxide supports, through sol-gel preparations, can provide advantages that the respective single oxides (SiO2, TiO2) cannot. Silica-titania mixed oxides have been studied extensively. However, few studies have been done on their use as active metal supports. Furthermore, catalysts containing active metals supported over Si:Ti mixed oxides have not been prepared in the manner of this invention nor do they use the same materials.
The above-cited references are hereby incorporated by reference.
The present invention includes a catalyst, a method for its preparation and a method for using the catalyst.
The present invention includes a sol-gel supported catalyst for the dehydrogenation of lower alkanes, the catalyst comprising at least one active metal and at least one promoter metal attached to a sol-gel mixed oxide support. The sol-gel mixed oxide support arising from the polymerization of at least one precursor. The active metal and the promoter metal having been attached to the sol-gel mixed oxide support by the active metal and the promoter metal having been co-precipitated with the precursor of the sol-gel mixed oxide support.
Lower alkanes, as used herein, may typically include those alkanes of C(1-5) carbons.
The active metal, as used herein, may be any metal adapted to bring about oxidative dehydrogenation activity and may preferably include molybdenum or vanadium. The active metal may be present in any amount that is effective and may include anywhere from 1 to 70% by weight of the finished catalyst and more particularly from 1 to 20% by weight.
The promoter metal (also called the alkali promoter), as used herein, is selected from the group consisting of alkali metals, alkaline earth metals, rare earth metals, and mixtures thereof. The promoter metal is preferably selected from metals from Group IA of the Periodic Table, in particular, the promoter metal may include alkali metals such as Li, Na, K, Rb, or Cs.
The active metal is supported over a mixed metal oxide support and may preferably include a silica-titania mixed oxide support (i.e. SiO2xe2x80x94TiO2). The support mixed metal oxide molar ratio may range from 0:1 to 1:0, with a Si:Ti ratio of 1:1 being the most preferred.
Furthermore, promoter metal loading has been on a molar basis and has ranged from 0 to above 2-alkali/active metal molar ratios.
Also included within the scope of the present invention is a method of producing a sol-gel supported catalyst. The method comprising: obtaining a sol-gel precursor solution comprising at least one silicon alkoxide and at least one titanium alkoxide in a solution, adding to the sol-gel precursor solution at least one active metal-containing precursor in aqueous solution and at least one promoter metal-containing precursor in solution; and allowing the silicon alkoxide and the titanium alkoxide to become polymerized to form a sol-gel while allowing the active metal-containing precursor and the promoter metal-containing precursor to precipitate.
Active metal-containing precursors, as used herein, may be selected from the group consisting of ammonium heptamolybdate, molybdenum isopropoxide, molybdic acid, silicomolybdic acid, molybdenum chloride, molybdenum oxide, vanadium chloride, vanadium oxyalkoxides, vanadium acetylacetonate, vanadium pentoxide, vanadium acetate, pure V and Mo powders and other molybdenum or vanadium precursors.
Promoter metal-containing precursors, as used herein, may be selected from the group consisting of carbonates, nitrates, hydroxides, chlorides and molybdates of alkali, alkaline earth and rare earth metals (i.e. X2MoO4 where X is any of the above-mentioned metals).
The catalyst may be prepared in a solvent. The solvent may be any solvent that can dissolve titanium or silicon alkoxides. The solvent may be selected from alcohol, hexane, benzene or other polar-aprotic solvents to name a few. If alcohol is used, it may comprise pure or mixed alcohols selected from the group consisting of methanol, ethanol, propanol, iso-propanol, and butanol.
Finally, the present invention includes a method of dehydrating lower alkanes to produce lower alkenes using a catalyst. The method comprising the steps: (a) obtaining a sol-gel supported catalyst, the catalyst comprising at least one active metal and at least one promoter metal attached to a sol-gel mixed oxide support. The sol-gel mixed oxide support arising from the polymerization of at least one precursor thereof, the active metal and the promoter metal having been attached to the sol-gel mixed oxide support by the active metal and the promoter metal having been co-precipitated with the precursor of the sol-gel mixed oxide support; and (b) bringing into contact with the catalyst at least one lower alkane for sufficient time and at sufficient temperature so as to allow the lower alkane to be dehydrogenated.
The precursor of the sol-gel mixed oxide support, as used herein, may be any liquid alkoxide compound and is preferably silicon and titanium alkoxides. These include Si(OR)4 and Ti(OR)4, where R can be CH3, C2H5, linear C3H7, branched C3H7 (isopropyl) or C4H9.